High Tension Steel Plate with Small Acoustic Anisotropy and with Excellent Weldability and Method of Production of Same

ABSTRACT

The present invention provides a high tension steel plate of a tensile strength of 570 MPa or more with a small acoustic anisotropy and with excellent weldability tensile strength and a method of production of steel plate predicated on the high productivity accelerated cooling stop process, which steel sheet is a high tension steel plate with a value of 0.045%≦Nb+2Ti≦0.105%, A=(Nb+2Ti)×(C+N×12/14) satisfying 0.0025 to 0.0055 and with a steel composition containing bainite in an amount of 30 vol % or more and pearlite+martensite-austenite constituent (MA) in an amount of less than 5 vol %. The steel is heated to 1200° C. or more, rough rolled at 1020° C. or more, then rolled at a cumulative reduction rate from over 920° C. to less than 1020° C. of 15% or less and a cumulative reduction rate from 860 to 920° C. of 20 to 50%, then starting accelerated cooling giving a cooling rate of 2 to 30° C./sec at 800° C. or more, stopping the accelerated cooling at 600 to 700° C., then cooling by 0.4° C./sec or less.

TECHNICAL FIELD

The present invention relates to a method of production of high tensionsteel plate with a small acoustic anisotropy and with excellentweldability having a tensile strength of the 570 MPa or more with a highproductivity without requiring off-line heat treatment. The steel of thepresent invention is used for thick plate, steel pipe, or steel shapesas structural members of bridges, ships, building structures, marinestructures, pressure vessels, penstocks, line pipes, and other weldedstructures.

BACKGROUND ART

High tension steel plate of a tensile strength of the 570 MPa or moreused for structural members of bridges, ships, building structures,marine structures, pressure vessels, penstocks, line pipes, etc. isrequired to feature not only strength, but also toughness andweldability and in recent years is often particularly required tofeature weldability with large heat input. Numerous studies have beenmade in the past to improve these characteristics. The composition andproduction conditions of such steel plate are disclosed in for exampleJapanese Patent Publication (A) No. 53-119219, Japanese PatentPublication (A) No. 01-149923, etc. These relate to methods ofproduction comprising rolling steel plate, then reheat quenching itoff-line and further reheating it for tempering heat treatment. Further,for example, Japanese Patent Publication (A) No. 52-081014, JapanesePatent Publication (A) No. 63-033521, Japanese Patent Publication (A)No. 02-205627, etc. disclose technology relating to production byquenching steel plate on-line after rolling, that is, so-called directquenching. These require off-line tempering heat treatment both withreheat quenching and direct quenching. However, the requirement of aheat treatment process off-line inevitably ends up obstructing theproductivity, so to raise the productivity, a so-called “as-rolled”method of production eliminating tempering heat treatment and notrequiring off-line heat treatment is desirable.

Technology relating to the as-rolled method of production has beendisclosed in several publications, for example, Japanese PatentPublication (A) No. 54-021917, Japanese Patent Publication (A) No.54-071714, Japanese Patent Publication (A) No. 2001-064723, JapanesePatent Publication (A) No. 2001-064728, etc. These relate to theaccelerated cooling stop process which stops the accelerated coolingafter rolling the steel plate. Thus uses accelerated cooling to quenchthe steel down to the transformation temperature or less to obtain ahardened structure and stops the water cooling in a state of arelatively high temperature after transformation to shift to the gradualcooling process to obtain the tempering effect by this gradual coolingprocess and thereby eliminate the reheat tempering. However, theseproduction technologies all require controlled rolling under arelatively low temperature for obtaining toughness and strength. Thetemperature where the rolling is finished is around 800° C., so time isrequired for waiting for the temperature and the productivity cannot besaid to be high. On the other hand, in particular in bridges, buildings,and other applications, a small acoustic anisotropy is required sinceanisotropy affects the precision of the angle beam ultrasonic testing ofthe weld zone. In controlled rolling finishing the rolling at a 800° C.or so temperature, texture is formed, so the steel plate becomes largerin acoustic anisotropy and therefore will not necessarily match theseapplications.

Further, the above Japanese Patent Publication (A) No. 2001-064728discloses technology for production of high tension steel plate having atensile strength of 570 MPa or more by the accelerated cooling stopprocess. However, in this patent, V is considered to contribute toprecipitation strengthening even at the gradual cooling stage afterstopping the water-cooling midway, but the inventors' studies foundthat, as explained later, V has a slower precipitation rate in thegradual cooling stage after stopping the water-cooling midway comparedwith Nb and Ti and is not that effective for strengthening. With thiscomposition of ingredients, a stable strength probably cannotnecessarily be stably obtained.

Further, Japanese Patent Publication (A) No. 2002-053912 discloses anas-rolled process not performing water cooling after rolling. Since thisdoes not involve controlled rolling at a low temperature, the acousticanisotropy does not become large, but instead, to obtain strength, theamounts of addition of alloys of Cu, Ni, Mn, etc. become greater andtherefore there is a problem with economy.

DISCLOSURE OF INVENTION

Therefore, the present invention has as its object to obtain hightension steel plate of a tensile strength of 570 MPa or more with asmall acoustic anisotropy and with excellent weldability by aneconomical composition of ingredients with little amounts of addition ofalloys and a high productivity method of production predicated on anaccelerated cooling stop process. The thickness of the steel platecovered is up to 100 mm.

There are several means for strengthening high tension steel, but themethod of utilizing of precipitation strengthening by carbides ornitrides of Nb, V, Ti, Mo, and Cr etc. enables strengthening withrelatively small amounts of alloy ingredients. At that time, to obtain alarge amount of precipitation strengthening, it becomes important toform a precipitate with coherence with the base material.

In the accelerated cooling stop process, at the state of rolling, thesteel structure is austenite. The accelerated cooling after the end ofthe rolling causes this to transform to a bainite or ferrite or otherferrite base material structure. The precipitate formed in the austeniteduring rolling loses its coherence with the ferrite base material andbecomes smaller in strengthening effect after transformation. Further,the precipitate formed at an early stage of the rolling becomes coarserand becomes a factor lowering the toughness. Therefore, it is importantto suppress precipitation during the rolling and cause precipitation asmuch as possible in the bainite or ferrite structure at the stage ofgradual cooling after stopping the water-cooling. If a process involvingwater cooling, then reheating for tempering heat treatment, thetemperature and time for precipitation can be sufficiently secured, solarge precipitation strengthening can be easily obtained. As opposed tothis, in the case of the accelerated cooling stop process without reheattempering, precipitation can be expected during the gradual coolingafter the water-cooling is stopped, but to obtain a hardened structure,the water-cooling stop temperature has to be made a low temperature to acertain extent, so both the temperature and time for precipitation arerestricted. This is generally disadvantageous for precipitationstrengthening. From this, as explained above, the as-rolled process hasa high productivity, but requires a larger amount of alloy elements forobtaining the same strength as the thermal refinement process orrequires controlled rolling at a low temperature.

Therefore, the inventors engaged in in-depth studies on a method forutilizing precipitation strengthening to the maximum extent so as toobtain a high strength predicated on the high productivity acceleratedcooling stop process and without adding a large amount of alloy elementsor controlled rolling at a low temperature.

First, to clarify the precipitation behavior in the process of gradualcooling after stopping the water-cooling, the inventors studied in depththe relationships between the rates of precipitation of carbides,nitrides, and carbonitrides of the various alloy elements in a bainiteor ferrite structure and the amount of precipitation strengthening andthe temperature and holding time. As a result, in a bainite or ferritestructure or their mixed structures, the rates of precipitation of Nbcarbonitrides and Ti carbides are faster than those of V and otherelements and these form precipitates with coherence with the basematerial, so the amount of strengthening is large. In particular, therate of precipitation is fast and the amount of strengthening is largein the 600° C. to 700° C. temperature range. Further, when making jointuse of Nb and Ti or Nb, Ti, and Mo for composite precipitation, due tothe synergistic effect, even with short time holding, precipitatescoherent with the base material finely disperse and large precipitationstrengthening can be obtained.

However, if the amounts of addition of Nb and Ti are too large, theprecipitates formed tend to become coarse and the number of precipitatesconversely becomes smaller, so the amount of precipitation strengtheningfalls. The rates of precipitation and the form of precipitation ofcarbides, nitrides, and carbonitrides of Nb and Ti in austenite andferrite are greatly affected by the amounts of addition of Nb and Ti andby the amounts of C and N. The inventors engaged in various experimentsand analyses and obtained the discovery that the rates of precipitationand the form of precipitation of carbides, nitrides, and carbonitridesof Nb and Ti can be organized well by the parameterA=([Nb]+2×[Ti])×([C]+[N]×12/14) and that keeping this value within acertain range enables sufficient precipitation during the gradualcooling after stopping water-cooling while suppressing precipitationduring rolling. That is, the greater the amounts of addition of Nb andTi, the smaller the amounts of addition of C and N must be made. If thevalue of A is too small, the rate of precipitation in ferrite becomesslower and sufficient precipitation strengthening cannot be obtained.Conversely, if the value of A is too large, the rates of precipitationof carbides, nitrides, and carbonitrides in the austenite become toofast, the precipitates become coarser, and the amounts of coherentprecipitation during the gradual cooling after stopping the acceleratedcooling become insufficient, so the amount of precipitationstrengthening falls. Further, Si also has an effect on the rate offormation of carbides, so a certain range of amount of addition isnecessary.

These precipitation strengthening effects are greatly influenced by thestructure. With a bainite structure, as compared with ferrite, it iseasy to maintain the dislocation density and other worked structures. Topromote fine coherent precipitation, the sufficient presence ofdislocation, deformation zones, or other precipitation sites in theworked structure acts extremely effectively. According to the studies ofthe inventors, to obtain sufficient strengthening, a single bainitephase or a mixed structure of bainite and ferrite with a volume fractionof bainite of 30% or more is necessary. Further, pearlite,martensite-austenite constituent (MA), etc. precipitate at the phaseinterfaces resulting in a smaller strengthening effect and a drop intoughness etc., so the sum of the volume fractions of pearlite andmartensite-austenite constituent (MA) has to be suppressed to 3% orless.

The inventors further studied the specific production conditions forobtaining the maximum extent of precipitation strengthening and obtainedthe following discoveries.

The precipitation of Nb and Ti at the rolling stage is promoted byrolling strain, so the rolling conditions in the high temperature regionof austenite, the so-called rough rolling conditions, have a largeeffect on the final precipitation strengthening effect. Specifically,finishing the rough rolling in the temperature range of 1020° C. or moreand avoiding rolling as much as possible in the temperature range of1020° C. to 920° C. are requirements for suppressing precipitationduring rolling. However, if finishing all rolling in the temperaturerange of 1020° C. or more, almost no worked structure will remain afterthe accelerated cooling is stopped due to recovery andrecrystallization, so dislocation, deformation zones, and otherprecipitation sites will not be sufficiently present and sufficientprecipitation strengthening cannot be obtained. Therefore, it isessential to perform the necessary and sufficient rolling in the not yetrecrystallization temperature range, then quickly perform theaccelerated cooling after rolling. Specifically, relatively lightrolling with a cumulative reduction rate of 20% to 50% is performed inthe range of 920° C. to 860° C. Under these conditions, it is possibleto suppress unnecessary precipitation of Nb and Ti and leave suitableprecipitation sites even after stopping the water-cooling stop. Further,under these conditions, no strong texture is formed, so the acousticanisotropy also does not become great.

The temperature of stopping the water-cooling in the accelerated coolingstop process is made a temperature of the center of the sheet thicknessof 600° C. to 700° C. advantageous for the precipitation of Nb and Ti.To obtain a steel structure with a volume fraction of bainite of 30% ormore even with t his stopping temperature, it is necessary to limit thecomposition of ingredients of the steel to the later explained specificrange and ensure at cooling rate of 2° C./sec to 30° C./sec inaccelerated cooling. Further, it is essential to heat the slab at a hightemperature for dissolution of the Nb and Ti. A heating temperature of1200° C. or more is necessary.

The discovery obtained here is the new idea of controlling on-line theprecipitation of carbides or carbonitrides of Nb and Ti during therolling including the high temperature range, during the acceleratedcooling, and until the gradual cooling process after stopping thecooling. A precipitation strengthening equal to or greater than that ofthe conventional thermal refining process can be realized by anaccelerated cooling stop process not requiring off-line heat treatment.

Further, according to this production process, it is possible to keepdown the weld crack sensitivity index Pcm(Pcm=[C]+[Si]/30+[Mn]/20+[Cu]/20+[Ni]/60+[Cr]/20+[Mo]/15+[V]/10+5[B]:where [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] mean themass % of C, Si, Mn, Cu, Ni, Cr, Mo, V, and B) of the composition of thesteel material and possible to provide a steel material with a highwelding heat affected zone toughness even with a large heat input andwith excellent weldability.

The gist of the present invention is as follows:

(1) A high tension steel plate of a tensile strength of 570 MPa or morewith a small acoustic anisotropy and with excellent weldability,characterized by having a composition of steel containing, by mass %,

-   -   C: 0.03% to 0.07%,    -   Si: 0.1 to 0.6%,    -   Mn: 0.8 to 2.0%,    -   Al: 0.003% to 0.1%,    -   Nb: 0.025 to 0.1%,    -   Ti: 0.005 to 0.1%,    -   [Nb]+2×[Ti]: 0.045 to 0.105%    -   N: over 0.0025% to 0.008%,        containing Nb, Ti, C, and N in a range where the value A        expressed by the A of the following equation (1) is 0.0022 to        0.0055, and containing a balance of Fe and unavoidable        impurities, having a steel structure with a volume fraction of        bainite of 30% or more and a sum of volume fractions of pearlite        and martensite-austenite constituent (MA) of less than 5%:

A=([Nb]+2×[Ti])×([C]+[N]×12/14)  (1)

-   -   where [Nb], [Ti], [C], and [N] mean the mass$ of Nb, Ti, C, and        N

(2) A high tension steel plate of a tensile strength of 570 MPa or morewith a small acoustic anisotropy and with excellent weldability as setforth in (1), characterized in that said steel sheet further contains,by mass %, one or more of

-   -   Mo: 0.05% to 0.3%,    -   Cu: 0.1% to 0.8%,    -   Ni: 0.1% to 1%,    -   Cr: 0.1% to 0.8%,    -   V: 0.01% to 0.03%,    -   W: 0.1% to 3%,    -   B: 0.0005% to 0.005%,    -   Mg: 0.0005% to 0.01%,    -   Ca: 0.0005% to 0.01%.

(3) A high tension steel plate of a tensile strength of 570 MPa or morewith a small acoustic anisotropy and with excellent weldabilitycharacterized by heating a slab having a composition of ingredients asset forth in (1) or (2) to 1200° C. to 1300° C., rough rolling it at atemperature range of 1020° C. or more, then hot rolling it by acumulative reduction rate of 15% or less in the temperature range ofless than 1020° C. to over 920° C. and by a cumulative reduction rate of20% or more in the temperature range of 920° C. to 860° C., thenstarting accelerated cooling by a cooling rate of 2° C./sec to 30°C./sec from a temperature of 800° C. or more, stopping said acceleratedcooling at a temperature of the center of plate thickness of 700° C. to600° C., then cooling by a cooling rate of 0.4° C./sec or less.

BEST MODE FOR WORKING THE INVENTION

Below, the reasons for limitation of the ingredients and method ofproduction of the present invention will be explained.

C is an important element forming carbides and carbonitrides with Nb andTi and becoming a principal factor in the strengthening mechanism of thesteel of the present invention. If the amount of C is insufficient, theamount of precipitation during the gradual cooling after the acceleratedcooling is stopped becomes insufficient and strength cannot be obtained.Conversely, even if in excess, the precipitation rate in the austeniteregion during rolling becomes faster. As a result, the amount ofcoherent precipitation during the gradual cooling after the acceleratedcooling is stopped becomes insufficient and strength cannot be obtained.Therefore, the amount of C is limited to 0.03% to 0.07% in range.

Si is an element necessary as a deoxidizing element in steel making andalso has an effect on the rate of precipitation of carbides. Suitableaddition of Si has the effect of suppressing the precipitation ofcarbides in the austenite region during rolling. To achieve this object,Si is added in an amount of 0.1% or more, preferably 0.3% or more.However, if added over 0.6%, the precipitation rate becomes too slow.Further, the welding heat affected zone is made to drop in toughness, so0.6% is made the upper limit.

Mn is an element necessary for improving the hardenability and obtaininga single bainite phase or obtaining a mixed structure of bainite andferrite with a bainite percentage of 30% or more. To obtain this object,0.8% or more is necessary, but if added over 2.0%, sometimes the matrixmaterial is made to drop in toughness, so the upper limit is made 2.0%.

Al is added in the usual range as a usual deoxidizing element, that is,0.003% to 0.1%.

Nb and Ti are important elements forming NbC, Nb(CN), TiC, TiN, Ti(CN),or their composite precipitates and composite precipitates of these withMo and thereby forming main elements of the strengthening mechanism ofthe steel of the present invention. In the accelerated cooling stopprocess, to obtain a sufficient composite precipitate, suitable-additionconsidering the precipitation rate is necessary. That is, Nb is 0.025%or more, preferably 0.035% or more, and Ti is 0.005% or more,conditional on 0.045%≦([Nb]+2×[Ti])≦0.105% and, whenA=([Nb]+2×[Ti])×([C]+[N]×12/14), the value of A being 0.0022 to 0.0055(where, [Nb], [Ti], [C], and [N] respectively mean the mass % of Nb, Ti,C, and N). Note that the upper limit values of Nb and Ti arerespectively made 0.1%.

Mo improves the hardenability, forms a complex precipitate with Nb andTi, and greatly contributes to strengthening, so 0.05% or more is added.However, if excessively added, the welding heat affected zone toughnessis inhibited, so the amount added is made 0.3% or less.

N bonds with Ti to form TiN. When TiN finely disperses, its pinningeffect suppresses the coarsening of the structure of the welding heataffected zone and improves the welding heat affected zone toughness, butif the N is insufficient, the TiN becomes coarser and a pinning effectcannot be obtained. To make the TiN finely disperse, N is added in anamount over 0.0025%, preferably over 0.004%. Further, if excessivelycontaining N, conversely sometimes it causes the matrix material to dropin toughness, so the upper limit is made 0.008%.

Cu has to be added in at least 0.1% to exhibit its effect when added asa strengthening element, but even if added over 0.8%, the effect doesnot increase in proportion to the amount of addition and if excessivelyadded, sometimes impairs the welding heat affected zone toughness, sothe amount is made 0.8% or more.

Ni has to be added in at least 0.1% when added to raise the matrixmaterial toughness, but if excessively added, sometimes inhibits theweldability. It is also an expensive element, so the upper limit ofaddition is made 1%.

Cr has the effect of raising the hardenability in the same way as Mn andof facilitating obtaining a bainite structure. For this purpose, 0.1% ormore is added, but if excessively added, impairs the welding heataffected zone toughness, so the upper limit is made 0.8%.

V has the effect of increasing the precipitation strengthening andhardenability a certain extent though having less of a strengtheningeffect compared with Nb and Ti. To obtain this effect, addition of 0.01%or more is necessary. If excessively added, a drop in the welding heataffected zone toughness is caused, so even when added, the amount ismade less than 0.03%.

B increases the hardenability. When added to obtain strength, additionof 0.0005% or more is necessary, but even if added over 0.005%, theeffect does not change, so the amount of addition is made 0.0005% to0.005%. By adding one or both of Mg and Ca, it is possible to formsulfides or oxides and improve the matrix material toughness and weldingheat affected zone toughness. To obtain this effect, Mg or Ca has to beadded in amounts of 0.0005% or more. However, if excessively added over0.01%, coarse sulfides or oxides are formed, so conversely the toughnessis lowered in some cases. Therefore, the amounts of addition are 0.0005%or more and 0.01% or less, respectively.

As unavoidable impurities other than the above ingredients, P and S areharmful elements causing the matrix material to drop in toughness, sothe amounts are preferably small. Preferably, P is made 0.02% or less,while S is made 0.02% or less.

Next, the method of production will be explained.

To cause sufficient solid solution of Nb and Ti, the heating temperatureof the slab at the time of rolling must be made 1200° C. or more.However, even if the heating temperature exceeds 1300° C., the effect ofdissolution does not change that much and the energy cost becomeshigher, so the heating temperature of the slab at the time of rolling ismade 1200° C. to 1300°.

In the rolling, to suppress precipitation of Nb and Ti during rolling asmuch as possible, after the rough rolling at a suitable reduction rateat a temperature range of 1020° C. or more, rolling in the range of lessthan 1020° C. to over 920° C. is performed by a cumulative reductionrate of 15% or less. Further, to obtain a worked structure necessary andsufficient as a precipitation site, the rolling is performed in a rangeof 920° C. to 860° C. by a cumulative reduction rate of 20% to 50%. Withthese rolling conditions, formation of texture is suppressed, so theacoustic anisotropy does not become large.

To suppress recovery of the worked structure and precipitation afterwork, soon after the finish of rolling, accelerated cooling isperformed. The cooling is performed by water cooling from 800° C. ormore under conditions of a cooling rate at the center part in the platethickness of 2° C./sec to 30° C./sec. To make the volume fraction ofbainite 30% or more, a 2° C./sec or higher cooling rate is necessary. Tomake the sum of the volume fractions of pearlite andmartensite-austenite constituent (MA) less than 3%, the cooling rate ismade 30° C./sec or less. The water cooling is stopped midway so that thetemperature at the center of the plate thickness becomes 700° C. to 600°C., then cooling rate is made 0.4° C./sec or less by natural cooling.The object is to secure sufficient temperature and time forprecipitation of Nb and Ti and composite precipitation of these andcomposite precipitation with Mo. If the water-cooling stop temperatureis too high, a bainite structure becomes hard to obtain, theprecipitation becomes slower at a low temperature, and sufficientstrengthening cannot be obtained.

The steel of the present invention is used in the form of thick plate,steel pipe, or steel shapes as structural members of bridges, ships,building structures, marine structures, pressure vessels, penstocks,line pipes, and other welded structures.

EXAMPLES

Slabs obtained by producing steel of the compositions of ingredientsshown in Table 1 were processed under the production conditions shown inTable 2 and Table 3 to 20 to 100 mm thick steel plates. Among these, 1-Ato 14-N are steels of the present invention, while 15-O to 43-A arecomparative examples. In the tables, underlined numerical valuesindicate ingredients or production conditions outside the scope of thepatent or characteristics not satisfying the following target values.

The results of measurement of the tensile strength, welding heataffected zone toughness, and acoustic anisotropy for these steel platesare shown in Table 2. The tensile strength was measured by obtaining aNo. 10 rod test piece defined in JIS Z2201 and testing it by the methoddefined in JIS Z2241. The matrix material toughness was evaluated byobtaining an impact test piece defined in JIS Z2202 from the center ofthickness of the plate in the direction perpendicular to the rollingdirection and finding the fracture appearance transition temperature(vTrs) by the method defined in JIS Z2242. The welding heat affectedzone toughness was evaluated by the absorption energy at −20° C. (vE⁻²⁰)of an impact test piece defined in JIS Z2202 given a thermal cyclecorresponding to submerged arc welding with an amount of input heat of20 kJ/mm. For a steel material with a thickness of 32 mm or less, theoriginal thickness steel material was prepared. For a steel materialwith a thickness of over 32 mm, a steel plate reduced in thickness to 32mm was prepared. The V-shaped abutted part was welded by large inputheat submerged arc welding with an amount of input heat of 20 kJ/mm. Animpact test piece defined in JIS Z2202 was taken so that the notchbottom ran along the fusion line and evaluated by the absorption energy(vE⁻²⁰) at −20° C. The acoustic anisotropy was evaluated in accordancewith the Japanese Society for Non-Destructive Inspection standardNDIS2413-86. A sound speed contrast of 1.02 or less was evaluated as asmall acoustic anisotropy. The target values of the characteristics area yield strength of 450 MPa, a tensile strength of 570 MPa or more, avTrs of −20° C. or less, a vE⁻²⁰ of 70 J or more, and a sound speedcontrast of 1.02 or less.

Examples 1-A to 14-N all have yield strengths over 450 MPa, tensilestrengths over 570 MPa, welding heat affected zone toughnesses vE⁻²⁰over 200 J, and sound speed contrasts of 1.02 or less or small acousticanisotropies.

As opposed to this, Comparative Example 15-O has a low C, ComparativeExample 16-P has a high C, Comparative Example 17-Q has a low Si,Comparative Example 19-S has a low Mn, Comparative Example 21-U has alow Mo, Comparative Example 23-W has a low Nb, Comparative Example 25-Yhas a low Ti, Comparative Example 27-AA has a value of the parameter A(A=([Nb]+2×[Ti])×([C]+[N]×12/14)) of less than 0.0025, ComparativeExample 37-A has a low heating temperature, Comparative Example 40-A hasa high cumulative reduction rate in the range from 920° C. to 860° C.,Comparative Example 41-A has a small plate thickness center coolingrate, Comparative Example 42-A has a high accelerated cooling stoptemperature, and Comparative Example 43-A has a low accelerated coolingstop temperature, so their tensile strengths were less than 570 MPa.

Comparative Example 18-R has a high Si, Comparative Example 22-V has ahigh Mo, Comparative Example 24-X has a high Nb and an Nb+2Ti over0.105%, Comparative Example 26-Z has a high Ti and an Nb+2Ti over0.105%, Comparative Example 2 g-AC has a low N, Comparative Example31-AE has a high V, Comparative Example 32-AF has a high Cu, ComparativeExample 33-AG has a high Ni, Comparative Example 34-AH has a high Cr,Comparative Example 35-AI has a high Mg, and Comparative Example 36-AJhas a high Ca, so their welding heat affected zone toughnesses are low.

Comparative Example 20-T has a high Mn, Comparative Example 28-AB has avalue of the parameter A of over 0.005, and Comparative Example 30-ADhas a high N, so their matrix material toughnesses are low.

Comparative Example 38-A has a high cumulative reduction rate in therange from less than 1020° C. to over 920° C. and Comparative Example 3g-A has a low cumulative reduction rate in the range from 920° C. to860° C., so have low tensile strengths and have low welding heataffected zone toughnesses.

Comparative Example 3 g-A has a high cumulative reduction rate in therange from 920° C. to 860° C., so has a low tensile strength and a largeacoustic anisotropy.

TABLE 1 Steel Chemical composition (mass %) mat.. C Si Mn P S Cu Ni CrMo Al Nb Ti Nb + 2Ti A** V B Mg Ca N Pcm Inv. A 0.04 0.35 1.55 0.0080.003 0.07 0.03 0.051 0.014 0.079 0.0036 0.0053 0.134 steel B 0.04 0.311.28 0.008 0.005 0.27 0.03 0.038 0.015 0.068 0.0032 0.0056 0.132 C 0.030.37 1.52 0.006 0.002 0.13 0.04 0.049 0.022 0.093 0.0036 0.0071 0.127 D0.05 0.33 1.75 0.015 0.005 0.10 0.02 0.078 0.006 0.090 0.0049 0.00420.155 E 0.06 0.51 1.56 0.009 0.002 0.24 0.05 0.051 0.014 0.079 0.00520.0055 0.171 E 0.05 0.41 1.49 0.012 0.002 0.09 0.05 0.050 0.016 0.0820.0045 0.020 0.0042 0.146 G 0.04 0.35 1.54 0.008 0.003 0.15 0.01 0.0280.032 0.092 0.0045 0.0009 0.0077 0.143 H 0.04 0.22 1.42 0.005 0.004 0.410.12 0.01 0.054 0.015 0.084 0.0038 0.0041 0.133 I 0.07 0.33 0.98 0.0060.005 0.67 0.12 0.05 0.043 0.011 0.065 0.0049 0.0048 0.172 J 0.05 0.541.37 0.007 0.011 0.32 0.19 0.04 0.041 0.021 0.083 0.0048 0.0064 0.165 K0.04 0.41 0.88 0.012 0.007 0.24 0.02 0.046 0.008 0.062 0.0027 0.0150.009 0.0029 0.115 L 0.03 0.37 1.32 0.008 0.004 0.31 0.24 0.08 0.070.051 0.016 0.083 0.0030 0.0032 0.0055 0.149 H 0.04 0.33 1.54 0.0100.004 0.12 0.006 0.056 0.012 0.080 0.0037 0.0034 0.0050 0.136 N 0.040.30 1.38 0.007 0.003 0.24 0.02 0.057 0.009 0.075 0.0034 0.0019 0.00490.135 Comp. O 0.01 0.41 1.61 0.006 0.004 0.21 0.03 0.065 0.018 0.1010.0017 0.0058 0.118 steel P 0.09 0.25 1.32 0.005 0.004 0.22 0.03 0.0360.019 0.074 0.0071 0.0051 0.179 Q 0.06 0.11 1.58 0.005 0.003 0.16 0.050.037 0.019 0.075 0.0049 0.0050 0.153 R 0.06 1.22 1.37 0.002 0.002 0.180.03 0.048 0.015 0.078 0.0051 0.0049 0.181 S 0.07 0.38 0.52 0.006 0.0030.21 0.04 0.043 0.013 0.069 0.0053 0.0054 0.123 T 0.05 0.25 2.22 0.0050.006 0.19 0.03 0.047 0.011 0.069 0.0038 0.0048 0.182 U 0.05 0.44 1.510.003 0.005 0.02 0.03 0.048 0.021 0.090 0.0050 0.0049 0.142 V 0.06 0.511.39 0.060 0.003 0.51 0.05 0.047 0.011 0.069 0.0046 0.0055 0.181 W 0.060.55 1.55 0.010 0.005 0.24 0.03 0.015 0.022 0.059 0.0039 0.0055 0.172 X0.04 0.29 1.35 0.005 0.003 0.19 0.02 0.102 0.009 0.120 0.0054 0.00410.130 Y 0.06 0.30 1.52 0.004 0.004 0.25 0.03 0.061 0.002 0.065 0.00420.0046 0.163 Z 0.04 0.29 1.61 0.009 0.003 0.18 0.03 0.026 0.041 0.1080.0050 0.0051 0.142 AA 0.03 0.31 1.49 0.006 0.003 0.21 0.04 0.036 0.0110.058 0.0020 0.0042 0.129 AB 0.06 0.33 1.56 0.008 0.002 0.19 0.04 0.0520.023 0.098 0.0065 0.0055 0.162 AC 0.04 0.33 1.59 0.003 0.003 0.18 0.020.048 0.024 0.096 0.0041 0.0022 0.143 AD 0.04 0.34 1.57 0.004 0.003 0.190.03 0.049 0.016 0.081 0.0044 0.0021 0.143 AE 0.05 0.35 1.45 0.005 0.0050.23 0.02 0.039 0.015 0.069 0.0039 0.06 0.0052 0.156 AF 0.04 0.36 1.050.006 0.004 1.55 0.11 0.04 0.036 0.031 0.098 0.0045 0.0049 0.189 AG 0.060.35 1.32 0.050 0.030 1.81 0.12 0.04 0.045 0.017 0.079 0.0052 0.00480.176 AH 0.05 0.32 0.96 0.004 0.003 1.22 0.21 0.03 0.051 0.021 0.0930.0053 0.0061 0.184 AI 0.05 0.29 1.52 0.005 0.005 0.22 0.03 0.056 0.0180.092 0.0051 0.015 0.0044 0.150 AJ 0.04 0.37 1.41 0.003 0.002 0.15 0.040.051 0.016 0.083 0.0039 0.013 0.0055 0.133 *Pcm = C + Si/30 + Mn/20 +Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 + 5B **A = (Nb + 2Ti) × (C + N ×12/14)

TABLE 2 Cumulative Cumulative Cooling Water Welded Pro- reductionreduction rate at cooling Matrix heat Acoustic duction Heating rate rateplate stop Plate material affected anisotropy con- temp. at at 1020° C.at 920° C. to thickness temper- thick- Yield Tensile toughness zone(sound dition Steel rolling 920° C. 860° C. center ature ness strengthstrength vTrs toughness speed No mat. (° C.) (%) (%) (° C./sec) (° C.)(mm) (Mpa) (Mpa) (° C.) vE-20 (J) contrast) Inv. 1 A 1200 0 46 13 660 32520 617 −63 215 1.01 ex. 2 B 1220 0 45 30 620 20 532 635 −72 204 1.01 3C 1230 0 50 9 590 50 515 610 −61 213 1.01 4 D 1230 0 40 5 610 75 502 605−51 229 1.02 5 E 1220 0 35 3.5 570 100 522 633 −44 208 1.01 6 F 1230 040 25 650 20 546 650 −69 232 1.01 7 G 1200 10 46 16 630 32 489 589 −70202 1.01 8 H 1200 0 22 10 650 50 533 625 −50 212 1.00 9 I 1220 0 29 8580 75 509 607 −47 204 1.01 10 J 1250 0 36 7 600 75 518 625 −57 216 1.0111 K 1220 0 45 13 640 32 555 660 −60 231 1.02 12 L 1220 0 42 11 590 50512 613 −57 222 1.01 13 M 1230 0 40 14 670 32 542 628 −72 233 1.02 14 N1220 0 40 24 680 25 526 629 −68 218 1.01

TABLE 3 Cumulative Cumulative Cooling Water Welded Pro- reductionreduction rate at cooling Matrix heat Acoustic duction Heating rate rateplate stop Plate material affected anisotropy con- temp. at at 1020° C.at 920° C. to thickness temper- thick- Yield Tensile toughness zone(sound dition Steel rolling 920° C. 860° C. center ature ness strengthstrength vTrs toughness speed No mat. (° C.) (%) (%) (° C./sec) (° C.)(mm) (Mpa) (Mpa) (° C.) vE-20 (J) contrast) Comp. 15 0 1220 0 40 13 66032 427 543 −72 215 1.02 ex. 16 P 1220 0 42 13 630 32 444 563 −25 1101.02 17 Q 1220 0 38 18 640 32 426 551 −30 128 1.01 18 R 1250 0 35 22 64032 468 602 −39 43 1.01 19 S 1220 0 46 10 600 40 433 555 −66 222 1.02 20T 1220 0 33 21 630 32 537 677 −5 125 1.01 21 U 1220 0 40 12 590 40 444546 −79 204 1.01 22 V 1220 0 45 24 660 32 498 667 −24 37 1.01 23 W 12200 28 10 670 40 419 547 −66 215 1.02 24 x 1220 0 33 12 650 32 457 575 −3524 1.01 25 Y 1250 0 36 13 650 32 435 550 −80 220 1.02 26 z 1220 0 40 13660 32 468 602 −40 21 1.02 27 AA 1220 0 40 14 670 32 422 537 −52 2101.01 28 AB 1220 0 40 13 650 32 439 561 −24 18 1.01 29 AC 1220 0 40 14650 32 514 612 −21 51 1.01 30 AD 1220 0 38 13 640 32 527 630 −10 56 1.0131 AE 1230 0 32 11 680 40 518 617 −35 31 1.02 32 AF 1220 0 40 13 600 32531 635 −44 48 1.01 33 AG 1220 0 33 15 630 32 514 622 −80 46 1.02 34 AH1220 0 33 14 590 32 510 630 −26 19 1.02 35 AI 1220 0 33 16 660 32 511635 −8 27 1.01 36 AJ 1220 0 40 15 670 32 520 625 −10 50 1.02 37 A 1150 033 18 670 32 441 546 −65 232 1.01 38 A 1220 33 45 16 660 32 418 525 −4052 1.02 39 A 1220 0 10 17 670 32 438 557 −22 22 1.02 40 A 1220 0 66 15630 32 443 551 −38 95 1.04 41 A 1220 0 36 1 620 32 405 522 −45 202 1.0242 A 1220 0 33 22 740 32 444 541 −35 110 1.02 43 A 1220 0 40 19 480 32446 563 −30 100 1.02

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to obtain hightension steel plate with a small acoustic anisotropy, with excellentweldability, and with a tensile strength of 570 MPa or more up to aplate thickness of 100 mm by economical ingredients with little amountsof addition of alloys and by a high productivity as-rolled method ofproduction. The effect on the industry is extremely great.

1. A high tension steel plate of a tensile strength of 570 MPa or more with a small acoustic anisotropy and with excellent weldability, characterized by having a composition of steel containing, by mass %, C: 0.03% to 0.07%, Si: 0.1 to 0.6%, Mn: 0.8 to 2.0%, Al: 0.003% to 0.1%, Nb: 0.025 to 0.1%, Ti: 0.005 to 0.1%, [Nb]+2×[Ti]: 0.045 to 0.105% N: over 0.0025% to 0.008%, containing Nb, Ti, C, and N in a range where the value A expressed by the A of the following equation (1) is 0.0022 to 0.0055, and containing a balance of Fe and unavoidable impurities, having a steel structure with a volume fraction of bainite of 30% or more and a sum of volume fractions of pearlite and martensite-austenite constituent (MA) of less than 5%: A=([Nb]+2×[Ti])×([C]+[N]x×12/14)  (1) where [Nb], [Ti], [C], and [N] mean the mass$ of Nb, Ti, C, and N
 2. A high tension steel plate of a tensile strength of 570 MPa or more with a small acoustic anisotropy and with excellent weldability as set forth in claim 1, characterized in that said steel sheet further contains, by mass %, one or more of Mo: 0.05% to 0.3%, Cu: 0.1% to 0.8%, Ni: 0.1% to 1%, Cr: 0.1% to 0.8%, V: 0.01% to 0.03%, W: 0.1% to 3%, B: 0.0005% to 0.005%, Mg: 0.0005% to 0.01%, Ca: 0.0005% to 0.01%.
 3. A high tension steel plate of a tensile strength of 570 MPa or more with a small acoustic anisotropy and with excellent weldability characterized by heating a slab having a composition of ingredients as set forth in claim 1 to 1200° C. to 1300° C., rough rolling it at a temperature range of 1020° C. or more, then hot rolling it by a cumulative reduction rate of 15% or less in the temperature range of less than 1020° C. to over 920° C. and by a cumulative reduction rate of 20% or more in the temperature range of 920° C. to 860° C., then starting accelerated cooling by a cooling rate of 2° C./sec to 30° C./sec from a temperature of 800° C. or more, stopping said accelerated cooling at a temperature of the center of plate thickness of 700° C. to 600° C., then cooling by a cooling rate of 0.4° C./sec or less. 